There are frequency bands that can be shared by other kinds of wireless communication systems, one of which is a television white space (TV WS). The TV WS is an idle frequency band remained as an analog TV becomes digitalized, and this band corresponds to a spectrum of 54˜698 MHz assigned for TV broadcasting. If a licensed device having priority to use a relevant frequency domain does not use this frequency band, an unlicensed device can use this frequency band.
If the IEEE 802.11 is applied to the TV WS, there is an advantage that coverage is noticeably extended due to the spectrum characteristics of the TV WS. However, in general, if the coverage is extended, the number of STAs within the coverage is definitely increased. As the number of STAs increases, a method for flexibly managing users, i.e., scalability becomes an object of concern. Further, since various wireless communication systems and various unlicensed devices coexist, a problem of coexistence occurs. If distributed coordination function (DCF) and enhanced distributed cannel access (EDCA) protocols of IEEE 802.11 are applied to this environment, the problem of scalability may grow worse.
The DCF is a channel access mechanism used in IEEE 802.11 and based on carrier sense multiple access/collision avoidance (CSMA/CA). Also, the EDCA corresponds to a competition-based medium access method among channel access models proposed in a hybrid coordination function (HCF) provided by extension of a general medium access control protocol. The HCF is a protocol provided by IEEE 802.11e proposed to guarantee quality of service (QoS).
Further, if an IEEE 802.11 protocol is applied to the TV WS, it is expected that overlapping base service sets (BSSs) are rapidly increased. For example, if a user voluntarily installs an access point (AP) supporting the TV WS, coverage of the AP is so large that overlapping zone may increase.
When trying to use the TV WS, an unlicensed equipment utilizes geo-location database to acquire an available channel a corresponding zone. Also, to solve a problem of coexistence between unlicensed equipments, there is a need for a signaling protocol such as a common beacon frame or the like.
A terminal for the IEEE 802.11 TV WS refers to an unlicensed equipment that operates using IEEE 802.11 MAC and PHY in the TV WS. Hereinafter, a station (STA) and AP operating in the TV WS will be called a WS STA and a WS AP, respectively.
The WS STA has to provide a protection function for an incumbent user. That is, if the incumbent user such as a microphone is in of use, the WS STA has to stop the use of a channel for protecting the incumbent user. To this end, the WS STA requires a spectrum sensing mechanism. The spectrum sensing mechanism includes an energy detection method, a signature detecting method, etc. It is determined that the incumbent user is in of use if strength of a received signal is equal to or higher than a certain value, or it is determined that the incumbent user is in of use if a DTV preamble is detected. However, a WS communication mechanism that reflects the incumbent user protection function of a common WS may have troubles in maintaining quality of service and securing reliability since an ascertain and standby time is taken. Accordingly, a more-detailed WS communication mechanism is required to maintain the quality of service and secure the reliability of the WS STA.